Issue33

#33 , tested. 

BTW, I tried to get back the [previous three test Apps](https://github.com/hupili/hk_census_explorer/blob/4c2ffafbfc5cfd898900eb12ca3756b5b7244de9/frontend/app/scripts/controllers/test.js#L43) and can not fit into current `CensusAPI`. Is it a bit over specified? i.e. some operations like `joinData` or `joinGroups` only apply to a type of applications. Should we make a thin service only to stablize interface and leave manipulation to Apps?